Determination of 4.438 MeV γ-ray to neutron emission ratio from a 241Am–9Be neutron source
Introduction
All (α,n) neutron sources are made from α-emitting isotope and suitably low-Z targets. 9Be is the most important target because it offers the highest neutron yield. A stable alloy can be formed between Be and actinide α-emitters of the form MBe13, where M represents the actinide metal (Knoll, 2000). The (α,n) neutron sources are used for many application like activation analysis (Pinault, 1998; Shahriari and Sohrabpour, 2000), calibration source (Croft, 1989), and industrial applications (Akaho et al., 2001; Jonah et al., 1992), because they are relatively inexpensive, compact, portable, and reasonably constant although they yield a rather low output.
241Am–9Be source has a long half-life period (432.7 yr) and is therefore used in many laboratories especially as a neutron and gamma calibration source (Norman, 2001). It is not only a common neutron source but also a γ source that produces 4.438 MeV photons.
Neutron and photon reaction channels are,
There are also other channels for this reaction but their probabilities are very low. Fig. 1 shows the energy level and the electromagnetic transition of 12C. 9Be(α,n)12C reaction in 241Am–9Be source, almost leads to either ground state or first-level excited state, [channels (a) and (b)] (Norman, 2001; Croft, 1989). The intensity of 4.438 MeV γ-ray to neutron ratio, R=Sγ/Sn, is a very important characteristic of (α,n) sources, specially in mixed n–γ field spectroscopy (Pal et al., 1998) and dosimetry (Milman et al., 2001).
In the present work, the γ-ray pulse height spectrum of a 241Am–9Be source was measured using a NaI(Tl) detector. The pulse height spectrum was calculated using the MCNP4C code (Briesmeister, 2000) and the ratio of 4.438 MeV γ-ray to neutron was calculated.
Section snippets
Measurements
The gamma pulse height spectrum of a 241Am–9Be source with 1.49×1011 Bq activity and background were measured by a cylinderical, 2″×2″, NaI(Tl) detector. Measuring time was 10 min, both spectra are shown in Fig. 2. These spectra are in agreement with those measured by Vega-Carrillo et al. (2002). The source was located 110±0.5 cm above the floor level, 78±0.5 cm from the center of the detector, and 165±0.5 cm from the nearest wall. In the measured pulse height spectrum the following peaks can be
Background and contamination in the 241Am–9Be pulse height spectrum
Due to cosmic radiation that continuously bombard the earth's atmosphere, the existence of natural radioactivity in the environment, and the presence of radioactive sources in the lab, all detectors record some background signal. Also, in gamma spectroscopy in mixed γ–n field, due to the interaction of neutrons, there is an on-line background.
So, the 241Am–9Be pulse height spectrum is contaminated by
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The normal background is due to sources present in the lab and the natural γ-ray emitters
Computations and discussion
To obtain the net spectrum of 4.438 MeV γ-ray, the three kinds of spectrum contaminations were subtracted from the experimental spectrum. Detector efficiency and pulse height spectrum were calculated using a 4.438 MeV gamma source instead of the neutron source. Then we normalized the smoothened Monte Carlo pulse height spectrum with the highest peak (3.927 MeV single scape peak) of the net spectrum. Fig. 4 shows a good agreement between them, so we can accept the total net counts, Nt integral of
Conclusion
The ratio of the 4.438 MeV γ-ray to neutron in the 241Am–9Be source was calculated using the experimental–computational approach. The correction of gamma spectrum due to the interaction of neutrons with the materials is possible only with a Monte Carlo simulation, although a low scatter room can reduce the effect. The present work demonstrates a useful approach using MCNP code that can be applied in many other fields.
Acknowledgements
The authors would like to thank Prof. G. Furlan, head of the TRIL program at ICTP, Trieste, Italy, for his contribution in this work.
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